ASSEMBLY AND METHOD FOR
MAGNETIZATION OF PERMANENT MAGNET
ROTORS IN ELECTRICAL MACHINES
BACKGROUND
[0001] The subject matter disclosed herein relates generally to electrical
machines, particularly to electrical machines having permanent magnet type rotors.
Specific embodiments relate to an assembly and method for magnetization of
permanent magnet segments in such rotors.
[0002] An electrical machine generally includes a rotor disposed within a
stator and is used either as a motor to convert electrical power to mechanical power or
as a generator to convert mechanical power to electrical power. Certain electrical
machines use permanent magnet type rotors that reduce the size and enhance the
overall efficiency of the machine. A permanent magnet rotor generally includes an
annular permanent magnet disposed over a rotor spindle. In certain embodiments, the
permanent magnet is a monolithic hollow cylindrical member. In larger machines, the
permanent magnet is generally formed by assembling a plurality of permanent
magnets around a rotor spindle. High-speed electrical machines may also include a
holding ring or a retaining ring around the permanent magnet assembly to prevent
fracturing and scattering of the permanent magnet assembly by centrifugal forces.
[0003] The permanent magnet segments are often magnetized prior to
assembly on the rotor spindle. In one technique, the permanent magnet segments are
cut and shaped from larger unfinished magnet blocks, after which the segments are
magnetized individually in a solenoid coil. In some applications, especially in larger
machines, magnetization of the permanent magnet segments is achieved via a
magnetization vector proposed by K. Halbach (also known as Halbach
magnetization), which, when applied to the surface of the permanent magnets, results
in a more sinusoidal shaped flux distribution within the electrical machine, thereby
reducing AC harmonic losses and reducing torque ripple, vibration, and acoustic
noise. The permanent magnet segments are subsequently bonded to the rotor spindle.
[0004] Assembly of rotors from pre-magnetized permanent magnet segments
is often a cumbersome process, especially in larger electrical machines, as it may
involve substantial forcing and aligning to position and restrain the energized
permanent magnet segments.
[0005] A technique for one-step magnetization has been disclosed in
commonly assigned Stephens US20060220484. However, it would still be desirable
to have a simpler and more efficient technique for magnetization of the electrical
machine rotors.
BRIEF DESCRIPTION
[0006] Briefly, in accordance with one embodiment, a magnetizer for a rotor
of an electrical machine is provided. The magnetizer comprises a magnetizing yoke
comprising multiple pole-pieces extending therefrom. At least some of the pole-
pieces comprise a cobalt alloy. The magnetizer also comprises a plurality of coils
wound around the pole pieces.
[0007] In another embodiment, a method of magnetizing a rotor of an
electrical machine is provided. The method comprises assembling a plurality of
magnets around a rotor spindle and positioning a magnetizer circumferentially around
the plurality of magnets. The magnetizer comprises at least one cobalt alloy pole-
piece.
DRAWINGS
[0008] These and other features, aspects, and advantages of the present
invention will become better understood when the following detailed description is
read with reference to the accompanying drawings in which like characters represent
like parts throughout the drawings, wherein:
[0009] FIG. 1 illustrates an exemplary embodiment for a magnetizer for
magnetizing a rotor;
[0010] FIG. 2 illustrates an exemplary embodiment showing a cross-sectional
view of a magnetizer adapted for a small sized rotor;
[0011] FIG. 3 illustrates another exemplary embodiment showing a cross-
sectional view of a magnetizer adapted for a large sized rotor; and
[0012] FIG. 4 illustrates a cross-sectional view of exemplary magnetizing
coils for a magnetizer.
DETAILED DESCRIPTION
[0013] The embodiments disclosed herein provide an assembly and method
for magnetizing an electrical machine rotor. A simple magnetization method is
provided for an entire assembled rotor in a multi-pole magnetizer. FIG. 1 illustrates a
magnetizing fixture 10 (herein referenced as a "magnetizer") for a rotor 12 of an
electrical machine. Magnetizer 10 comprises a magnetizing yoke 16 (or "core")
which comprises a plurality of pole-pieces 34, 38, 42, and 46 extending therefrom and
a plurality of magnetizing (or "excitation") coils 18, 20, 22, and 24 wound around the
pole-pieces of the magnetizing yoke. In a first embodiment, at least some of the
plurality of pole-pieces comprise a cobalt alloy. In a second embodiment, the
plurality of coils comprise hollow tube coils (described in more detail with reference
to FIG. 4) configured to allow a flow of a coolant therethrough. In a third
embodiment, the pole-pieces each comprise a fixed pole-piece portion 34 and an
interchangeable pole-piece portion 36 physically coupled to the fixed pole-piece
portion. The interchangeable pole-piece portions are of different dimensions to adapt
to different sized rotors. The first, second, and third embodiments may be used
individually or combined in any desired manner.
[0014] Magnetizer 10 is used, for example, for magnetizing the permanent
magnet rotors of high-speed electric motors. When magnetizer 10 incorporates
interchangeable components, magnetizer 10 may be used with different rotor
diameters. FIG. 1 illustrates a rotor 12 that is securely positioned around a rotor
spindle 14 for magnetization of the rotor's permanent magnet segments 15 by the
magnetizer 10. In one example, the permanent magnet segments 15 are situated in a
Halbach arrangement wherein each magnetic pole of the rotor is formed from several
magnets pieces and wherein the orientations of the magnet pieces are progressively
swept from radially tangential at the sides of the pole to radially normal at the center
of the pole.
[0015] The magnetizing yoke 16 may comprise any structurally suitable
material capable of use in magnetizing the rotor. In one example, yoke 16 comprises
steel laminations. Although there are some advantages expected from fabricating
both the yoke and the pole-pieces from a cobalt alloy, such embodiments are
expensive. A steel yoke in combination with cobalt alloy pole-pieces results in a
structure expected to provide more efficient magnetization than all-steel structures
and at less expense than all-cobalt alloy structures.
[0016] Magnetizing coils 18, 20, 22, and 24 may comprise any appropriate
electrically conductive material, for example copper and a more specific example
being provided in the discussion of FIG. 4 below. The number of magnetizing coils is
generally chosen to be equal to the number of poles of the rotor. Although four poles
and four coils are illustrated, any suitable number may be used. The coils 18, 20, 22
and 24 are energized by a power source (not shown). When energized, the
magnetizing coils 18, 20, 22, and 24 produce a magnetic flux through the magnetizing
poles that magnetizes the rotor 12.
[0017] As described above, the magnetizing poles may be integral to the yoke
but are more typically fabricated from separate pole-pieces. In a more specific
embodiment, the magnetizing poles each comprise a fixed pole-piece 34, 38, 42, or 46
coupled to the magnetizing yoke 16 and an interchangeable pole-piece 36, 40, 44, or
48 coupled to a respective fixed pole-piece 34, 38, 42, and 46. These embodiments
are for example only. In other embodiments, the interchangeable pole-pieces may be
coupled directly to the yoke or may be coupled to integral fixed pole-pieces (not
shown) extending from the yoke, for example. Coupling may be by any appropriate
approach with one illustrated embodiment including keys and slots, shown generally
by reference numeral 35. In an exemplary embodiment these fixed pole-pieces and
the interchangeable pole-pieces comprise a cobalt alloy. The pole-pieces may also be
shaped to magnetize Halbach magnet arrangements in the rotor.
[0018] FIG. 2 and FIG. 3 illustrate two exemplary embodiments showing
partial cross-sectional views of example segments of magnetizer 10 used for different
sized rotors. Differently sized interchangeable pole-pieces are provided for the
appropriate rotor diameter. FIG. 2 illustrates a cross-sectional view 50 that illustrates
a yoke portion 16 and a small diameter rotor 52. The interchangeable or replaceable
pole-piece is shown by reference numeral 54 and the fixed pole-piece is shown by
reference numeral 56. The coil is shown by the reference numeral 58.
[0019] FIG. 3 similarly illustrates a cross-sectional view 60 of another
exemplary embodiment of the magnetizer 10 used for a large diameter rotor 62. The
interchangeable pole-piece 54 of FIG. 2 is replaced by the pole-piece 64 to
accommodate a large sized rotor. The fixed pole-piece 56 and the coil 58 are same as
shown in FIG. 2. The multi-sizing feature of the magnetizer (by virtue of
interchangeable pole-pieces) reduces manufacturing capital investment, since most of
the fixture can be commonly used over the different sizes of rotors of any electrical
machine product line. FIG. 2 and FIG. 3 illustrate racetrack shaped coils 58, however
other suitable shaped coils such as but not limited to saddle coils as shown in FIG. 1
may be used.
[0020] In one embodiment, the fixed and interchangeable pole-pieces as
described in FIGs. 1 -3 are fabricated from cobalt alloy laminations. In one exemplary
embodiment, the cobalt alloy is a cobalt-iron alloy that has a greater flux density
capability than conventional steel laminations and therefore increases the
magnetization effect on the permanent magnets of the rotor. The cobalt alloy in one
specific example comprises 49% cobalt, 49% iron, and 2% vanadium with respect to a
total weight of the cobalt alloy. In another example, the cobalt alloy comprises cobalt
in a range of 48 weight percent to 50 weight percent, vanadium in a range of one
weight percent to two weight percent, and balance iron. In another example the cobalt
alloy comprises cobalt in a range of from about 45 weight percent to about 55 weight
percent with respect to the total weight of the cobalt alloy. In another example, the
cobalt alloy further comprises iron in a range of from about 45 weight percent to
about 55 weight percent with respect to the total weight of the cobalt alloy. In another
example, the cobalt alloy comprises vanadium a range of from about 1 weight percent
to about 4 weight percent with respect to the total weight of the cobalt alloy.
[0021] In another embodiment hollow tube coils are used to allow a flow of a
coolant as shown in FIG. 4. In a more specific example, the hollow tube coils 66
include copper conductors 68 that carry a liquid coolant such as water, oil, or a
cryogenic fluid such as liquid nitrogen or liquid neon 70 as the coolant. The
conductors have thin turn insulation layers (not shown) around each conductor 68 and
a coil side insulation 72. The cooling lowers the coil resistance and minimizes the
power source requirement.
[0022] The different embodiments of the magnetizer as described herein
provide improved magnetization of an electrical machine rotor. The magnetizer
described herein can be used for a wide range of electrical machinery, including
motors, and particularly including large high-speed synchronous machines for gas line
compressors, aerospace motors, aerospace generators, marine propulsion motors,
marine power generators among others.
[0023] While only certain features of the invention have been illustrated and
described herein, many modifications and changes will occur to those skilled in the
art. It is, therefore, to be understood that the appended claims are intended to cover
all such modifications and changes as fall within the true spirit of the invention.

CLAIMS
1. A magnetizer (10) for a rotor (12) of an electrical machine, the
magnetizer comprising:
a magnetizing yoke (16), wherein the magnetizing yoke comprises a plurality
of pole-pieces (34, 36, 38, 40, 42, 44, 46, 48) extending therefrom, at least some of
the plurality of pole-pieces comprising a cobalt alloy; and
a plurality of coils (18, 20, 22, 24)wound around the pole-pieces.
2. The magnetizer of claim 1 wherein the cobalt alloy comprises 49%
cobalt, 49% iron, and 2% vanadium with respect to a total weight of the cobalt alloy.
3. The magnetizer of claim 1 wherein the cobalt alloy comprises cobalt in
a range of from 40 weight percent to 60 weight percent with respect to the total
weight of the cobalt alloy and iron in a range of from about 40 weight percent to
about 60 weight percent with respect to the total weight of the cobalt alloy.
4. The magnetizer of claim 1 wherein the cobalt alloy comprises cobalt in
a range of 48 weight percent to 50 weight percent with respect to the total weight of
the cobalt alloy, vanadium in a range of one weight percent to two weight percent
with respect to the total weight of the cobalt alloy, and balance iron.
5. The magnetizer of claim 1 wherein each of the pole-piece comprising
the cobalt alloy comprises:
a fixed pole-piece portion (34, 38, 42, 46); and
an interchangeable pole-piece portion (36, 40, 44, 48) physically coupled to
the fixed pole-piece, wherein at least the interchangeable pole-piece portion
comprises the cobalt alloy.
6. The magnetizer of claim 1, wherein the magnetizing yoke (16)
comprises a material different from the material of the pole-pieces (34, 36, 38, 40, 42,
44, 46, 48).
7. The magnetizer of claim 6 wherein the yoke material comprises steel.
8. The magnetizer of claim 1 wherein the plurality of coils comprise
hollow tube coils (68) configured to allow a flow of a coolant therethrough.
9. The magnetizer of claim 8 wherein the hollow tube coils comprise
copper coils.
10. The magnetizer of claim 8 further comprising liquid nitrogen situated
in the hollow tube coils.